Use of an aluminum electrode (a porous aluminum electrode) in which a porous layer made of a sintered layer of aluminum powder is laminated on a surface of an aluminum core material instead of aluminum foil subjected to etching treatment as an anode of an aluminum electrolytic capacitor has been developed. According to such an aluminum electrode, there is an advantage that an etching process using hydrochloric acid or the like is not necessary to be carried out (refer to Patent Literature 1). In addition, when the porous aluminum electrode is used, there are advantages that a sufficiently thick porous layer can be formed and the electrostatic capacitance can be increased due to a structure in which pores are complicatedly intricate.
However, when the porous aluminum electrode is used as anode foil for medium and high voltage, leakage current tends to be larger than that of the aluminum foil subjected to the etching treatment. The reason is considered to be, for example, the reasons described below with reference to FIG. 5. FIG. 5 is a view schematically illustrating the reason why the leakage current tends to become large when the porous aluminum electrode is used. FIG. 5(a) schematically illustrates the state of the porous layer 30 in the production process of the anode for an electrolytic capacitor and FIG. 5(b) schematically illustrates the state of aluminum powder 31 constituting the porous layer 30 in the production process of the anode for an electrolytic capacitor.
When the porous aluminum electrode is subjected to pure water boiling, a hydrated film 36 is formed on the surfaces of the porous layer 30 and the aluminum powder 31 illustrated in FIGS. 5(a) and 5(b). Such a hydrated film 36 is porous and voids 37 exist. In particular, in the case of a porous aluminum electrode, the voids 37 tend to be generated in the hydrated film 36 because the surface of the aluminum powder 31 constituting the porous layer 30 has higher reactivity with boiling pure water than that of the surface of the etching layer. Such voids 37 can be removed from the chemical formation film 38 by depolarization such as thermal depolarization treatment or the like when the chemical formation voltage is relatively low (for example, when the chemical formation voltage is less than 400 V) in a chemical formation step. However, when the chemical formation voltage is relatively high (for example, when the chemical formation voltage is 400 V or more), the voids cannot be sufficiently removed because the chemical formation film 38 is thick. Therefore, the chemical formation liquid left behind in the void 37 cannot be removed by pure water washing or the like. Consequently, when the chemical formation liquid expands during the subsequent chemical formation treatment or thermal depolarization treatment, defects 39 are generated in the chemical formation film 38 and thus the leakage current increases. Such a phenomenon is remarkable particularly when the chemical formation is carried out in an aqueous solution containing an organic acid or a salt thereof. More specifically, the organic acid in the chemical formation liquid remaining inside the void 37 burns/explodes due to the heat generated during the chemical formation or the heat of the thermal depolarization treatment. This breaks the chemical formation film 38 and the porous layer 30. As a result, the leakage current increases.
On the other hand, in order to reduce the leakage current, a method for producing aluminum foil for an electrolytic capacitor including a step of depositing an organic acid on the surface of a hydrated film after pure water boiling has been developed (Patent Literature 2).